The present invention relates to an X-ray lithography scanning mirror. More particularly, the invention relates to a scanning single mirror used in X-ray lithography.
X-ray lithography is a technique which uses photons in the approximately 800 to 1800 eV energy range for the fabrication of electronic and computer chips. A mask with the desired circuit pattern, or its inverse pattern, is located in front of the wafer and is irradiated by the X-rays. The exposed, or unexposed, areas are then chemically etched away, so that circuit features are imprinted on the silicon wafer. At present, the primary means of chip fabrication is through the use of photons in the visible and ultraviolet energies, up to approximately 10.sup.2 eV. The best line width resolution for this "optical" lithography is about 0.5 micron or micrometer, where 1 micrometer or micron equals 10.sup.-6 meter. At present, X-ray lithography has been used to achieve line widths as good as 0.3 micron in a research environment. Presumably, the next generation lithographic techniques, that is, using X-rays for mass production of chips, will yield line resolutions of 0.25 micron, or better.
The energy range for the X-rays used is typically approximately 800 to 1800 eV. The X-ray source which holds the most promise for generating photons of the required energy and intensity is a synchrotron. The synchrotron uses magnets to accelerate electrons along a specific orbit around the machine. At each point where magnets bend the beam of electrons to keep them in their orbit, photons from infrared energies, up to 10,000+ eV, are emitted. A small fraction of these photons are accepted into a beamline which is tangent to the radius of the bending angle in the machine. The beamline is an ultra high vacuum stainless steel pipe with equipment designed to filter the undesirable low and high energy photons. The beamline, therefore, serves as a means to permit mostly X-rays in the 800 to 1800 eV energy range to pass through to the "stepper", where the silicon wafers are awaiting irradiation.
Most of the sub-800 eV X-rays are eliminated as they emerge through the exit window on their way out of the beamline and into the stepper. Reflection of the X-ray beam from a mirror is the technique used to eliminate most of the X-rays which have energies higher than 1800 eV. Shaping the mirror as a curved surface permits it to be used to focus the X-ray beam to a desired beam spot shape. The advantage of focusing a large beam down to a smaller size is that the wafers in the stepper can be irradiated more quickly. The size of the field on the wafer to be irradiated is expected to be as large as 1" tall.times.2" wide. It would therefore be desirable to shape the beam spot so that it is at least as wide as the width of the field. The most common approach for irradiating the wafer is to take this wide and well, especially vertically, focused beam and either scan it across the face of the field, or use a stationary X-ray beam and scan the wafer in front of such beam.
Great difficulties are encountered at present in achieving 0.25 micron resolution with scanning steppers, because of vibration and alignment problems associated with their scanning. Apparently, a scanning mirror system will be required in the near future for the industrialization of chips with 0.25 micron features. The optics may therefore require an ability to scan the X-ray beam across the wafer as well as the ability to shape the beam properly and with the appropriate beam characteristics. The University of Wisconsin's Center for X-ray Lithography (CXRL), which is one of the primary research centers in this field, has recently developed a three-mirror system for use at their synchrotron, ALADDIN, which can achieve 0.25 micron resolution. This mirror system uses two focusing mirrors along with a plane mirror for scanning their beam. No single mirror approach is known which can both focus the beam and scan it while still maintaining the 0.25 micron resolution requirements over a field width of 50 mm. The scanning mirror of the invention is able to achieve 0.25 micron resolution, or better, for a 50 mm wide field while being able to scan the X-ray beam across a stationary wafer.
Silicon wafers are irradiated by placing a mask pattern, describing the circuit pattern to be etched into the silicon, in front of the wafer with the X-ray beam impinging its surface. The mask openings permit the X-rays to pass to the silicon for irradiation and the opaque parts of the mask attenuate most of the X-rays. This mask pattern is printed onto the wafer and later etched for chip fabrication. Ideally, the X-rays should be incident perpendicular to the mask and wafer, that is, with 0 divergence, to achieve the best line resolution. Due to the finite size of the source and the fact that an attempt is made to take a large angular acceptance of the X-ray emissions from the source and focus it to the desired size, the rays impinging on the mask will diverge or converge to a certain extent. The key is to minimize the angular deviation of the X-rays from the normal. The shape of the beam spot will have implications on the throughput of the system, but does not affect the lithographic line resolution of the system. That is, how many wafers can be irradiated in a given period of time. For this reason, the mirror design has been optimized to have good divergence properties at the expense of a non-rectangular beam spot shape, and therefore lower throughput.
The beam divergence/convergence deviation from a ray normal to the mask contributes to what is called the overlay error. The smaller the overlay errors, the better the line resolution one can get. One of the two causes of overlay error is called the horizontal magnification (X'). The rays of the mirror of the invention actually converge to the center of the beam spot along the horizontal plane. The divergence is greatest at the outermost parts of the beam along the horizontal. Thus, X' is approximately .+-.0.41 mrad at .+-.25 mm. Moving toward the bottom extreme, this divergence becomes almost .+-.0.06. Moving toward the top extreme, the horizontal divergence increases to .+-.0.8 mrad. Notably, the divergence at the top extreme is not nearly as large, small as it is, anyway, as it appears, since the outer horizontal parts of the beam are considerably higher than the top edge of the field. In reality, therefore, the maximum horizontal divergence is only a little greater than the value at the nominal position, since the top of the smile is already almost touching the top of the field. The maximum horizontal divergence is then approximately .+-.0.50 mrad at .+-.25 mm at the upper outer edges of the field. It should also be noted that the divergence quoted here is for an uncorrected system and with a line fit through the distribution to determine the average divergence as a function of horizontal position. If the distribution can be accurately described by some type of function such as a line, then some compensations may be made for a larger than desired divergence by modifying the fabrication of the mask. If the field size is only 25 mm wide, the horizontal divergence at the extremes, .+-.12.5 mm, at the nominal mirror angle position is only .+-.0.20 mrad and only about .+-.0.25 mrad in the worst case, with the mirror tilted to cover the top extremes of the field.
Another cause of overlay error is the vertical magnification or distortion. This relates the divergence of the X-rays in the vertical (Z) direction as a function of position along the width of the beam, horizontally. The uncorrected divergence at .+-.25 mm is approximately 0.4 mrad and only 0.1 mrad at .+-.12.5 mm. The shape of the distribution changes in width slightly for the three positions, but with only a maximum of about .+-.10% change in the divergence values. The entire distribution does shift considerably in Z', or the divergence of X-rays in the Z direction. This shift of the distribution in phase space is scan induced and is linearly related to the mirror angle as scanning is in the Z direction. This effect is inherent in any scanning mirror system, since when the angle of the X-ray beam is changed, the angle of the rays relative to the mask normal changes. Since this divergence is directly related to the angle of the scanning mirror, it can be compensated for in the mask fabrication and should not present any loss of line resolution.
The aforedescribed overlay errors affect the line resolution. Since a mask is used to cast a shadow of the pattern to be irradiated on the wafer, the gap distance between the mask and wafer becomes critical in defining how large a divergence is tolerable for achieving the desired resolution. Since the stepper will also have some errors associated with proper mask/wafer alignments and vibrations, there is that much less error budget available to the optics for achieving the desired resolution. Typical gap sized may range from as small as 10 microns to several tens of microns. The overlay error is computed as follows: EQU Overlay Error (in nm)=[(Horiz. Magn.).sup.2 +(Vert. Distort.).sup.2 ].sup.1/2 (in rad.).times.gap distance (in nm)
The principal object of the invention is to provide an X-ray lithography optical system which functions efficiently, effectively and reliably in fabricating a chip.
An object of the invention is to provide an X-ray lithography scanning mirror which functions efficiently, effectively and reliably in assisting an X-ray lithography optical system in fabricating a chip.
Another object of the invention is to provide an X-ray lithography scanning mirror of relatively simple structure which effectively reduces the overlay error in the fabrication of a chip.
Still another object of the invention is to provide an X-ray lithography scanning mirror which provides a substantially uniform distribution.
Yet another object of the invention is to provide an X-ray lithography scanning mirror which provides a distribution uniform to within .+-.5%.
Another object of the invention is to provide an X-ray lithography scanning mirror of relatively simple structure which functions efficiently, effectively and reliably to effectively reduce the overlay error and provide a substantially uniform distribution in the fabrication of a chip.